GMAT
题目
Donna is a mountain biking enthusiast. One Saturday, she spent the morning biking up an uphill trail at an average speed of 20 kilometers per hour, and then returned by the same route in the afternoon at an average speed of 25 kilometers per hour. If the downhill trip in the afternoon took of an hour less than the uphill trek in the morning, how many kilometers did Donna ride each way?
解析
设唐娜上坡骑行的时间为\( x \)。
### 上坡行程：
速度 = 20
时间 = \( x \)
距离 = \( 20x \)（距离公式：距离 = 速度×时间）
### 下坡行程：
速度 = 25
时间 = \( x - \frac{3}{4} \)
距离 = \( 25x - \left(25 × \frac{3}{4}\right) \)
已知往返路线的距离相等，因此可列等式求解\( 20x \)（即单程距离）：
\[
20x = 25x - \left(25 × \frac{3}{4}\right)
\]
化简得：
\[
5x = 25 × \frac{3}{4} \quad \text{（两边同乘4得到\( 20x \)）}
\]
\[
20x = 25 × 3
\]
\[
20x = 75
\]
答案：E
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